The present invention relates to an image forming system and image forming method, particularly to an image forming system having a plurality of stackers and image forming method thereof.
In a widely known image forming apparatus, after an image has been formed on many sheets of paper, these sheets are separated into groups each consisting of a predetermined numbers of sheets, and are stacked. Such an image forming apparatus comprises an image forming section for forming an image on paper, and a stacking unit for separating the sheets of paper fed out of the image forming section, into several groups each consisting of a predetermined numbers of sheets, and are stacked (for example, see Patent Document 1 which represents Official Gazette of Japanese Patent Tokkaihei 5-155177 corresponding to U.S. Pat. No. 5,248,136). The stacking unit uses a sorter-based stacker, described in the Patent Document 1, for sorting a predetermined number of the sheets of paper as a single stacking unit, or a stacker designed merely for stacking a predetermined number of sheets, whereby these sheets are separated into groups each consisting of a predetermined numbers of sheets. To put it another way, when a sorter-based stacker is used, the destination of the sheets is switched to another bin after a predetermined number of sheets have been loaded in one bin, thereby allowing the sheets to be separated into groups consisting of a predetermined number of sheets and to be stacked. In the meantime, when a plurality of the stackers designed merely for stacking is used, the destination of the sheets is switched to another stacker designed merely for stacking after a predetermined number of sheets have been loaded in one of these stackers, thereby allowing the sheets to be separated into groups consisting of a predetermined number of sheets and to be stacked.
If a jam such as a paper jam has occurred-on conveyance unit side, the jammed paper must be removed. Normally, an image is recorded on this paper, so image formation subsequent to clearing the jammed paper is restarted from the paper having been removed. This arrangement allows the sheets of paper to be stacked, without the order being affected, even after clearing of the jammed paper.
Since there is an increasing demand for higher speed in recent years, an image forming apparatus has been developed to reduce the timed intervals for conveyance of sheets and to minimize the loss of time in conveyance, wherein an image is formed continuously on a plurality of sheets. A higher speed can be achieved by reducing the timed interval for conveyance. However, if the timed interval for conveyance has been reduced below the difference in time (difference in conveyance time) between the time for conveying the paper to the destination closer to the image forming section (first destination) and the time for conveying it to the farther destination (second one), then the order of the sheets to be stacked may be disturbed. Such a failure will cause a jam when the destination of conveyance is switched from the first destination to the second one. This trouble is likely to occur when processing of clearing the jammed paper is carried out.
To put it in greater details, if the destination is switched after conveyance of the N-th sheet to the second destination, the (N+1)th and subsequent sheets are sent to the first destination. In this case if the interval timed for the conveyance is smaller than the difference in conveyance time, then the (N+1)th sheet will be sent to the first destination before the N-th sheet reaches the first destination and will be stacked in some cases. If this occurs, at least the (N+1)th sheet is stacked at the first destination even when paper has jammed at the second destination. When image formation is restarted after jammed paper has been cleared, the image will be formed sequentially on sheets, starting from the image corresponding to the n-th sheet. Since the image corresponding to (N+1)th sheet is formed on the new sheet even after the restart, the image corresponding to the (N+1)th sheet will be stacked in duplication. To prevent this duplication, not only the n-th sheet after clearing of jammed paper, but also the (N+1)th sheet stacked at the first destination must be removed. Such a procedure involves the correctly stacked sheet to be removed, and may make a user less willing to remove the paper or concerned about stacking after image formation. It will take a lot of time if an attempt is made to remove the duplicated sheet after checking against the original.
If the interval timed for conveyance is reduced by using a higher speed, the operation of switching section for conveyance to any one of the multiple stackers may not be terminated within the interval timed for conveyance.